The objective of this proposal is to extend our current studies which have suggested a role of mammalian liver in the renewal of erythrocyte purine nucleotides. We have shown by in situ perfusion of rabbit liver that adenosine, formed in the liver cell and released by that cell, is readily taken up by perfused human erythrocytes and converted to adenine nucleotides. Adenosine released from labeled rabbit liver slices also serves as a precursor of human and rabbit erythrocyte adenine nucleotides in vitro. These findings suggest a mechanism for the turnover of adenine nucleotides in the mammalian erythrocyte, known to lack the de novo pathway for purine nucleotide biosynthesis, and for the maintenance of the concentrations of the adenine nucleotides, essential for the viability of the cell.